Cranes are frequently used in the offshore oil industry in the construction of offshore drilling and production platforms. Conventionally, cranes have been designed to lift loads above the surface of the sea and the vertical length of lift is between the boom point and the sea surface. Cranes conventionally use blocks mounted to a boom with the wire rope reeved through the blocks to determine the crane lifting capacity in conjunction with tackle design and wire rope safety factors.
Several blocks may be provided along a crane boom, conventionally referred to as the "main" block, "auxiliary" block, "whip" block, etc. The blocks are sized to match the load which might be lifted at the selected reach of the crane boom. Thus, an outboard block would have fewer sheaves, with concurrent decrease in crane capacity and increase in travel ratio relative to inboard blocks.
The production of oil from offshore locations has moved progressively to deeper locations. A need has arisen for cranes with a capability of lifting over extended distances, including submerged locations adjacent to the sea floor.
A first problem arises in accommodating the length of wire rope required for the extended vertical travel. The anticipated loads for the submerged operations might be handled by blocks other than the main block. However, these auxiliary blocks are generally connected with a single hoist drum which provides insufficient wire rope storage and does not provide a balanced hoist reeving capability. It will be appreciated that only limited space is available with the rotating crane machinery cab mounted on the flotation barge to accommodate an increase in hoist capabilities for these auxiliary blocks. Costs of modifying existing equipment can range from substantial to prohibitive.
Differential horizontal forces across a block give rise to a torque tending to twist the ropes connecting the load to be lifted with the boom. The amount of angular rotation is a function of the length of the lift and is not a particular problem for conventional lifts above the sea surface. However, for the extended lifts required to handle equipment adjacent the sea floor, a degree of rotation can arise which effectively prevents operation of the crane due to friction forces between the ropes which are rotated into binding contact.
Both static and dynamic forces can arise during the extended block travel. Sheaves rotate at different speeds within the block, fastest adjacent the hoist rope leadout and slowest adjacent the stationary end of the rope. Thus, a gradient of friction forces arise across the block. Other horizontal forces can arise across the block tending to rotate the block such as, for example, forces arising from the rope offlead in passing between two parallel sheaves. The rope design itself, i.e. right or left lay, can introduce angular torsional forces.
The main block generally has adequate rope hoist capacity stored in the crane house for the long lifts of interest. However, the conventional reeving configuration to obtain the heavy lift capability above the sea surface does not enable the stored rope to be used for extended travel without re-reeving the entire block assembly. Further, conventional main blocks have a plurality of sheaves which tend to increase the differential forces acting to rotate the rope assembly. Further, in particular applications, the undersea block must cooperate with guide assemblies which direct the operation occurring on the sea floor. Conventional main blocks are simply too large to pass through guide assembly dimensions which are useful in correcting the underseas operation.
Thus, a need has arisen for a main block with increased versatility. It would be desirable to minimize horizontal forces generated within the block tending to rotate the wire ropes. It would be desirable to enable the speed and travel of the main block to be increased without having to re-reeve the block. It would further be desirable to enable the main block to be used with conventional guide assemblies. Additionally, it would be desirable to utilize only the active sheaves needed for the particular load to be lifted in order to minimize wear on the rope and the sheaves and to further minimize the required downhaul weight.
The disadvantages of the prior art are overcome by the present invention, however, and an improved apparatus is provided for configuring a main boom to accommodate a variety of load and travel conditions without the need to re-reeve the main block.